Zemanek, U.S. Pat. No. 3,369,626 discloses an ultrasonic tool for use in scanning the inner surface of an open well borehole or of casing in a borehole. The tool, which is commercially known as the "borehole televiewer", creates a high resolution picture of the inner surface under investigation. The borehole televiewer is used to "see" the inner surface under investigation through drilling mud or other borehole fluids. In an open borehole, the borehole televiewer provides a picture of the formations surrounding the borehole. In a cased borehole, the borehole televiewer provides a picture of the inner surface of the casing, which can be used to determine the condition of the inner surface.
The borehole televiewer uses a rotating ultrasonic transducer. The transducer serves as a transmitter, to generate acoustic waveforms, and a receiver, to receive the acoustic return. The acoustic return is caused by the reflection of the generated acoustic waveform from the inner surface under investigation. The acoustic return has two measured parameters, the time of travel of the acoustic return and the amplitude, which give an indication of the condition of the investigated surface.
The borehole televiewer measures the time of travel of the acoustic return with threshold detection circuitry located inside of the tool. Simple threshold detection is subject to error caused by cycle skipping. Cycle skipping occurs when the amplitude of the acoustic return varies and causes the threshold detection circuitry to trigger on different cycles within the acoustic return. Such inconsistent triggering, on different cycles, causes variations in the time of travel measurements.
In the prior art there are many techniques which determine the actual time of travel and amplitude of the acoustic returns. Because of the bandwidth limitation of the logging cable however, the borehole televiewer cannot implement any of these techniques; instead only the envelope of the acoustic return is transmitted to the surface.
What is needed is a method of measuring the time of travel of acoustic returns from an ultrasonic tool, which is not subject to cycle skipping.